1. Field of the Invention
The present invention relates to a liquid container in the form of an ink tank having a chamber for directly containing ink being print liquid for use in an inkjet printing apparatus.
2. Description of the Related Art
As a method for detecting an amount, or presence and absence, of ink reserved in an ink tank used in an inkjet printing apparatus, a method employing an optical technique is known. U.S. Pat. No. 5,689,290 discloses one configuration example for the method in which a light emitter and a light receiver are arranged in such a manner as to face each other with the ink tank in between, and whether or not light passes through the ink tank is detected. In addition, for example, U.S. Pat. No. 5,616,929, U.S. Pat. No. 7,172,259 and the like disclose a configuration in which a prism portion is provided to an inner wall of an ink tank. In this configuration, whether or not the light reaches a light receiver by being reflected from the prism portion is detected. This configuration utilizes a difference between deflection of the light when ink is in the ink tank and deflection of the light when ink is not in the tank.
FIGS. 3A and 3B show a basic configuration example of an ink tank equipped with a prism. FIG. 3A is a cross-sectional view showing the overall configuration of the ink tank, and FIG. 3B is a cross-sectional view taken along the IIIb-IIIb line in FIG. 3A. The ink tank includes an accommodating chamber 704 for a negative pressure generating member, and a liquid containing chamber 707. The ink tank is detachably mounted on an inkjet print head which is not shown. The accommodating chamber 704 accommodates the negative pressure generating member 701, and includes a liquid supply port 702 for the inkjet print head and an air communication portion 703 through which outside air is introduced along with consumption of ink contained. The liquid containing chamber 707 includes a communication portion 705 communicating with the accommodating chamber 704 for the negative pressure generation member, and forms a substantially hermetically enclosed space. The liquid containing chamber 707 is a container in which ink is directly reserved. A prism 700 is provided at a bottom face of the liquid containing chamber 707 so as to be used for detection of presence and absence of contained liquid (ink) 706.
FIG. 4 is a view illustrating a prism-type detection mechanism using the prism 700, and shows a positional relationship among the prism 700 provided at the bottom face of the ink tank, a light emitting element 808 for irradiating light to the prism 700 and a light receiving element 809 for receiving the light reflected from the prism 700.
As shown in FIG. 4, the prism 700 is molded integrally with a bottom face 810 of the ink tank. The cross section of the prism 700 has a shape in which two inclination faces 804 form an angle of 90° (FIG. 3B). For example, the basic wall thickness of the ink tank is 1.7 mm to 2.0 mm, and he vertex of the 90° angle of the prism 700 protrudes from an inner surface of the bottom face 810 toward the inside of the ink tank by a height of 3.2 mm.
The light emitting element 808 is outside and below the ink tank, and light therefrom incidents on the prism 700. When the ink tank contains sufficient ink, the ink is in contact with a surface of the prism 700. In such a case, incident light on the prism 700 travels optical paths indicated by (1) and (2′), and is thereby absorbed into the ink from the inclination face 804 of the prism. Accordingly, the light does not return to the light receiving element 809. In contrast, in a case where the ink in the ink tank has been consumed and the ink is not in contact with a surface of the prism 700, incident light on the prism 700 is reflected from the inclination surface 804 previously being an interface with the ink, and thereby travels optical paths indicated by (1) and (2), and then (3) to reach the light receiving element 809.
As described, whether light irradiated by the light emitting element 808 returns to the light receiving element 809 depends on whether the ink is in the ink tank, and presence and absence of ink can be detected based on this.
Note that the light emitting element 808 and the light receiving element 809 are usually provided to the main body of the printing apparatus. Generally, in an actual system, the presence or absence of ink is determined based on a threshold, considering influences such as background light. The ink presence or absence is detected based on whether an amount of the reflected light exceeds the threshold or not.
What is demanded in recent inkjet printing apparatuses is high-quality, high-speed image formation as well as reduction in costs of the apparatuses themselves and the running costs. The prism-type detection mechanism as described above is used for warning an occasionally occurring shortage of remaining ink, and is therefore not a factor of determining the basic performance of the printing apparatus, such as quality and speed. However, if printing must be redone due to running out of ink, consumables, such as ink and printing media, and time are wasted. If provided to the printing apparatus, the prism-type detection mechanism, which has a simple configuration, can be very effective in avoiding such a waste of consumables and time. In that sense, it can be said that the above-described prism-type detection mechanism is a reasonable mechanism for detecting the level or presence/absence of ink in the ink tank.
To perform such optical detection, ink drops left on the inner walls of the ink tank can be problematic. In other words, even when the ink is not in the ink tank, the light is not reflected from the prism if ink drops are present on an optical path for the optical detection. This hinders detection of ink shortage.
For example, U.S. Pat. No. 5,689,290 proposes a configuration for actively causing ink drops not to remain on the inner wall by providing the inner face of the ink tank with grooves through which the ink drops flow down. Moreover, Japanese Patent Laid-Open No. 2000-43287 proposes a technique of making adhesion of ink drops unlikely by water-repelling the inclination faces of the prism.
The above-described groove formation in the inner wall of the ink tank and water-repelling of the inclination faces of the prism are techniques for causing ink drops, which would affect optical detection, not to be present on the processed surface.
However, depending on the kind of ink contained (e.g., pigment ink), such countermeasures sometimes cannot shed the ink drops from the wall as expected. The water-repelling effects may vary, and satisfactory water-repelling effects might not be obtained.
Meanwhile, recent inkjet printing apparatuses have a higher printing speed with an increase in the number of nozzles and in the ejection frequency. Along with this, the consumption speed of ink is increased. Accordingly, the prism-type detection mechanism is required to make its detection quickly to be commensurate with the ink consumption speed. In reality, however, there is a time lag of several minutes or more.
FIG. 5 shows results of measurements of change in outputs of the light receiving element after the occurrence of ink shortage in an ink tank having the same prism and the same configuration. The output of the light receiving element corresponds to an amount of reflected light from the prism. What can be seen from FIG. 5 is that, although the amount of reflected light increases over time, each of the results shows a time lag between the occurrence of ink shortage and when the light amount reaches a high value, and there is variation among the results as well.
As shown in FIG. 6, the time lag occurs when ink drops 1004 exist for a certain period of time by randomly adhering to one inclination face 1002 and to the other inclination face 1003 after appearance of the prism 700 from an interface 1001 between ink and air. The higher the ink consumption speed, the more likely the ink drops 1004 adhere. The ink consumption speed very much depends on a percentage of ink dots in a printing surface of a printing medium (print duties).
In study over preparation for demands expected in the future in an inkjet printing apparatus capable of printing with even higher speed, the present inventors have reached the following findings.
Specifically, in regular character printing and photo printing, the print duties are relatively low, and oftentimes, the inkjet printing apparatus is used while taking stops between operations. Accordingly, an amount of ink adhering to the prism surface is little or, if there is any, very slight. Therefore, the ink amount detection is carried out without any problems, and it is very unlikely that the ink runs out during the printing operation.
On the other hand, in printing posters or banners, the print duties are high, and on top of that, the continuous printing operation oftentimes lasts a long period of time. In such a case, large ink drops might possibly adhere to the prism surface, which causes a time lag unignorable in the ink amount detection. If such a time lag occurs, ink runs out during the printing operation, and the consumables and time are consequently consumed wastefully.
A possible measure to solve the above problem is to improve the light amount of the light emitting element 808 and/or the sensitivity of the light receiving element 809. However, such improvement of the performances of the elements produces a harmful effect of increased costs for the elements. In addition, another harmful effect is the increased possibility of involuntary detection of background light, which is unrelated to ink consumption.
U.S. Pat. No. 6,454,400 proposes a configuration in which, in addition to the water-repelling of the prism surface as described earlier, a groove is formed around the prism in such a manner as to encircle the prism, and another groove extends from the encircling groove to a chamber accommodating an absorber. In this configuration, through the grooves, ink around the prism is actively introduced to the chamber accommodating the absorber. In this event, ink likely to remain on the prism inclination face is also brought along promptly. Thereby, ink is prevented from remaining on the prism surface in form of drops. This configuration is described as a method of making the ink drop adhesion unlikely, and this method has already been in practical use.
However, depending on the kind of ink contained (e.g., pigment ink), such a configuration still might not be able to shed the ink drops from the wall face as expected, and variation in the water-repelling effects may prevent satisfactory water-repelling effects from being obtained as described above.